To establish a seam on a fabric there exists, today, on the market a number of devices of different designs for performing lock stitches. On common home sewing machines an upper thread and a bottom thread on a bobbin in cooperation with a needle is used, in a known way, to bring the upper thread to perform a lock stitch on the fabric, which is sewn on the sewing machine.
A correct relation between the length of the upper thread and the length of the bottom thread of a stitch is desirable to accomplish a seam that looks decorative and holds a high quality. The proportion between the length of the upper thread and the bottom thread of each stitch depends on the relation between the tension of the upper thread and the bottom thread, respectively, during the forming of a knot that is made by upper thread and bottom thread and which constitutes a lock for a stitch in the seam.
To obtain the desired quality of a stitch, it is desirable that the knot of a stitch can securely be placed at the desired location in relation to the fabric. Usually, an optimal location of the knot is in the middle of the fabric as seen in a cross section of the extension of the fabric.
In prior art it is known to automatically adjust the present thread tension of the upper thread based on thread consumption of former stitches of the seam. Such a device is disclosed, as an example, in document U.S. Pat. No. 6,012,405. In this device the real thread consumption of the upper thread is measured be means of a decoder for consumed thread length after a completed stitch, whereby this information about real thread consumption for an already performed stitch is used to adjust the thread tension of a subsequent stitch to accomplish a correct relation of thread lengths between upper thread and bottom thread. This and other similar solutions presupposes that the amount of upper thread required for a present stitch is known in advance.
Document U.S. Pat. No. 4,967,679 discloses a solution to attain automatic control of thread feed, wherein a sewing machine for straight seams utilizes to thread portioning out in accordance with a requisite amount of upper thread, and which adjusts the thread tension of the upper thread at zigzag sewing. The thread portioning out of this solution is mechanically driven, wherein rolls which drives the thread are rotated synchronically with the driving mechanical members of the sewing machine. At zigzag sewing the thread tension is electrically set according to a manually predetermined value. In this described solution the thread supply is indirectly supervised and not through a direct reading of a prevailing deviation between actual thread consumption and a predetermined thread consumption per stitch.
During sewing, the calculated consumption of upper thread sometimes deviates from the actual consumption, which for example can depend on that the displacement of the fabric differ from the theoretically accurate. If, e.g. it is desired to portion out the thread by means of a thread portioner, automatically portioning out a calculated amount per stitch, instead of adjusting the tensile force in the upper thread by means of conventional friction braking, a precise information about a deviation between a calculated and an actual thread consumption is required already during the present stitch. An object with the present invention is to provide a device and a method to obtain this desired information.